1. Field of the Invention
The present invention relates to a vacuum pump used for a semiconductor manufacturing apparatus, an electron microscope, a surface analysis apparatus, a mass spectrograph, a particle accelerator, an atomic fusion experimental apparatus and so on, and more particularly, relates to a vacuum pump having a thread-groove pump mechanism portion for exhausting gas molecules by the interaction between a cylindrical surface of a rotor rotating at high speed and a fixed screw stator.
2. Description of the Related Art
In a process such as dry etching, chemical vapor deposition (CVD), or the like performed in a high-vacuum process chamber in semiconductor manufacturing step, a vacuum pump such as a turbo-molecular pump is used for producing a high vacuum in the process chamber by exhausting gas from the process chamber.
This type of turbo-molecular pump has a plurality of rotor blades on the outer periphery of a cylindrical rotor and a plurality of stator blades, which is located and fixed between the rotor blades, mounted in a pump case. The rotor is integrated with a rotor shaft. The turbo-molecular pump rotates the rotor shaft at high speed with a drive motor to thereby exhaust a gas sucked through a gas suction port to a lower gas discharge port by the interaction between the rotor blades rotating at high speed and the fixed stator blades, thereby evacuating the inside of the process chamber connected to the gas suction port to a high degree of vacuum.
Such a turbo-molecular pump has drawbacks in that when the backing pressure is increased to make the pressure of the rotor blades from a molecular flow pressure to a viscous flow pressure, the compressing efficiency of the rotor blades is rapidly decreased and the rotational resistance is increased to cause a significant decrease in performance and an increase in heat generation of the rotation body, and to increase in the power necessary for maintaining the rotation of the rotating body such as a rotor. Therefore, as a means for correcting the drawbacks, the turbo-molecular pump mechanism portion constituted by the rotor blades and the stator blades has a thread-groove pump mechanism portion including a cylindrical surface of the rotor and a thread groove at the back stage thereof, wherein the compressibility is increased by the interaction between the cylindrical surface of the rotor and the thread groove, so that the backing pressure of the rotor blades can be held low even when the backing pressure of the pump is increased; thus, a decrease in the compressibility of the whole pump is prevented.
In the compound-type turbo-molecular pumps having the thread-groove pump mechanism portion and the turbo-molecular pump mechanism portion, a uniform narrow gap is formed between the rotating body and the fixed body during the rest of the pump. Meanwhile, in a pressure region where the pressure is in an intermediate flow, when the mean free path of the molecules becomes less than a certain gap, a sealing effect of a small gap between the cylindrical surface of the rotating body and the thread groove rapidly decreases to reduce the compressing efficiency of the thread-groove pump mechanism portion, so that the gap is required to be set as small as possible.
However, because the gap during the rest of the pump is uniform, when the gap is set extremely narrow, the cylindrical rotating blades have a largest displacement due to a centrifugal force at the end of the cylinder when the pump is actually operated to rotate the rotating body of the rotor at high speed, so that the gap becomes small at the end of the cylinder and large at the opposite side thereof because of a stress applied to the blades during the operation of the pump.
The gap between the rotating body and the fixed body may be small at the end of the cylinder because of other external factors such as vibration from the exterior, thermal expansion due to an increase in the temperature of the rotating body, mechanical election tolerance, parts tolerance and so on, thus, causing a risk of contact between the rotating body and the fixed body at the end of the cylinder. A large gap at the opposite side thereof may decrease the sealing performance between the cylindrical surfaces of the rotating body and the fixed body to cause a significant decrease in the compressing efficiency of the thread-groove pump.
The present invention has been made to solve the above problems and the object thereof is to provide a highly-reliable vacuum pump capable of preventing a damage due to the contact between the cylinders of a high-speed rotating rotor and stators and preventing a decrease in the compressing efficiency of the pump by maintaining a sealing performance of them during the operation of the pump.
In order to achieve the above object, a vacuum pump according to the present invention comprises: a rotor shaft rotatably supported in a pump case having a gas suction port opened in the upper surface and a gas exhaust port opened in the lower side; a drive motor for rotating the rotor shaft; a rotor fixed to the rotor shaft and formed of a multiple cylinder having a plurality of cylinders with different diameters arranged concentrically with respect to the rotor shaft center; and a thread-groove pump mechanism portion including the plurality of cylinders of the rotor, a stator formed of a multiple cylinder having a plurality of cylinders alternately located between the cylinders and fixed in the pump case, and thread grooves cut in the walls of the stator facing the cylindrical surfaces of the rotor; wherein the gaps defined by the outer walls of the cylinders of the rotor and the stator walls and the gaps defined by the inner walls of the cylinders of the rotor and the stator walls are formed so as to increase with the distance from the rotor shaft center, and the gaps defined by the outer walls of the cylinders of the rotor and the stator walls are formed larger than gaps defined by the inner walls of the cylinders of the rotor and the stator walls.
In the vacuum pump according to the present invention, preferably, the gaps defined by the walls of the cylinders of the rotor and the stator walls are larger at the end of the rotor cylinders than at the base, and the mean value of the gap at the base of the rotor cylinders and the gap at the end of the rotor cylinders increases with the distance from the rotor shaft center.
In the vacuum pump according to the present invention, preferably, the gaps defined by the outer walls of the cylinders of the rotor and the inner walls of the stator are larger at the end of the rotor cylinders than at the base, and the gaps defined by the inner walls of the cylinders of the rotor and the outer walls of the stator are smaller at the end of the rotor cylinders than at the base.
The rotor may be formed of two members that are an inner cylindrical rotor having an inside diameter to surround a stator column and an outer cylindrical rotor having an inside diameter to surround the inner cylindrical rotor.
A mounting structure for the rotor and the rotor shaft may be a structure in which a disk-shaped mounting section of the inner cylindrical rotor is superposed to the lower surface of the collar of the rotor shaft and integrally fastened in the axial direction of the rotor shaft, and a disk-shaped mounting section of the outer cylindrical rotor is superposed to the upper surface of the collar of the rotor shaft and fastened in the axial direction of the rotor shaft.
The rotor may have a stage at the lower end of a cylindrical rotor body fastened in the axial direction of the rotor shaft, the stage having a small-diameter cylinder joined thereto, and a large-diameter cylinder is joined to the outer wall of the lower end of the rotor body.
The thread-groove pump mechanism portion may have thread grooves in the plurality of cylinder walls of the rotor and the stator walls having a flat cylindrical surface.
The pump case may further comprise therein a turbo-molecular pump mechanism portion including a plurality of rotor blades integrally provided on the outermost wall of the multiple cylinder of the rotor and a plurality of stator blades alternately located between the rotor blades and fixed in the pump case.